Split off the argument parser as well as the manifest parser into
helper functions. Drop the pylint hints from the main function now that
it is considerably smaller.
This extracts the CLI entrypoint into `main_cli.py` and prepares the
codebase for the introduction of additional entrypoints. This should
not contain any functional changes.
The idea behind this is to add `main_api.py` (and maybe more in the
future), which will be similar to `main_cli.py` but contain the
`osbuild-api` entrypoint. This will make all entrypoints nicely symetric
and the only difference will be `setup.py` selecting the right
entrypoint for each executable, as well as `__main__.py` selecting the
entrypoint for the module itself (which we will keep to the CLI for
compatibility).
